Increased optical disc capacities have been achieved by decreasing the size of the recording marks written on the recording tracks on the information recording surfaces of optical discs, by using laser beams with shorter wavelengths to record and reproduce information, and by using objective lenses with larger numerical apertures, thereby reducing the size of the focused spot formed by the objective lens on the focal plane.
For example, a CD (compact disc), with a disc substrate functioning as a light transmitting layer substantially 1.2 mm thick (including a transparent protective layer disposed on the information recording layer and a spacer layer), a laser beam wavelength of substantially 780 nm, and an objective lens with a 0.45 numerical aperture (NA), can provide a recording capacity of 650 MB. A DVD (digital versatile disc), with a light transmitting layer substantially 0.6 mm thick, a laser beam wavelength of substantially 650 nm, and a 0.6 NA, can provide a recording capacity of 4.7 GB. The higher density BD (Blu-ray disc: registered trademark), using an optical disc with a light transmitting layer substantially 0.1 mm thick, a laser beam wavelength of substantially 405 nm, and a 0.85 NA, is capable of providing the large recording capacity of 25 GB per layer. BD details are disclosed in, for example, non-patent reference 1 below.
Optical disc capacity can be increased by increasing the number of information recording layers per optical disc. In a multilayer optical disc having multiple information recording layers, however, there is a problem of interlayer crosstalk, in which the quality of the reproduced RF signal and the quality of servo signals such as the tracking error signal is degraded by the detection not only of light reflected from the information recording layer (referred to below as the ‘intended layer’) on which information is to be recorded or from which information is to be reproduced but also light reflected from information recording layers other than the intended layer (referred to below as ‘other-layer stray light’). Techniques for suppressing the above type of interlayer crosstalk are disclosed in, for example, JP 2011-86354 (patent reference 1).
Recently, high density recording systems that use optical discs in which there is formed a super-resolution functional layer having a nonlinear light absorbance characteristic or nonlinear light transmittance characteristic such that the refractive index varies locally with the light intensity have been under study in the optical recording field. In these high density recording systems, it is possible to reproduce marks smaller than the λ/(4NA) diffraction limit determined by the wavelength λ of the light and the numerical aperture NA of the focusing lens, which are optical elements of the optical disc device, by causing a super-resolution effect (a change in refractive index etc.) in a localized area of high light intensity or high temperature in the focused spot on the optical disc. An optical disc that generates this type of super-resolution effect is referred to as a super-resolution optical disc. Non-patent references 2 to 5, listed below, for example, may be cited as prior art references related to super-resolution optical discs.
For example, non-patent reference 2 in the list below discloses an optical disc having a super-resolution mask layer that generates a super-resolution effect. A change in refractive index occurs in this super-resolution mask layer as the super-resolution effect; the local area in which this super-resolution effect occurs is sometimes referred to simply as an ‘aperture’. Non-patent references 2 and 3 in the list below disclose optical discs of the Super-RENS (Super REsolution Nearfield Structure) type that generate a super-resolution effect. Non-patent reference 4 in the list below proposes an optical disc which is formed by a material having a nonlinear light absorbance characteristic or nonlinear light transmittance characteristic such that the refractive index changes with the light intensity. The term ‘super-resolution optical disc’ will be used below to designate all optical discs that generate the super-resolution effects noted above. A major feature of super-resolution optical discs is that they can make use of BD and other conventional optical disc reproducing techniques, making it possible to obtain downward compatibility with BDs and other existing optical discs, which is considered difficult with other high-capacity technology, e.g., with hologram recording systems and near-field optical recording systems.